Need help with diagnosis

Severe potassium deficiency, probably due to a nutrient lockout. Flush substrate with the appropriate pH, then fertilize with potassium, possibly add a bit of magnesium. For quicker help, boil 200g of banana peels (organic) in 500 ml of water, let it steep for 24 hours, then strain. Dilute this liquid 1:10 with water and spray it from below onto the leaves. Good luck!